1. Field of the Invention
The invention relates to an apparatus including an aperture through which electron beams are radiated onto a patterned photoresist film formed on a semiconductor substrate for forming a circuit pattern thereon, and a holder for fixedly supporting the aperture therewith. The invention relates also to a method of fabricating such an apparatus.
2. Description of the Related Art
There has been widely used photolithography for forming a small pattern on a wafer as a semiconductor substrate. In such photolithography, there has been employed a step and repeat exposure system (stepper) in order to expose a patterned photoresist film to light. The stepper first employed ultra-violet ray beams as a light source. However, in these days, a light source emitting a light having a smaller wavelength has been employed in order to form a pattern in a smaller size. For instance, g-ray having a wavelength of 436 nm in a mercury lamp, i-ray having a wavelength of 365 nm in a mercury lamp, and then, KrF eximer laser beam having a wavelength of 249 nm have been employed so far.
A resolution can be increased by using a light source emitting a light having a smaller wavelength. However, such a light source is accompanied with reduction in a depth of focus, which causes a problem that sharpness of a pattern is degraded due to blooming.
For this reason, electron beam exposure draws attention, because it has a significantly greater depth of focus than light exposure. The electron beam exposure makes it possible to form a more highly resolved pattern than the light exposure, but has a shortcoming that it has a low throughput.
However, there has been developed an exposure process which solves the above-mentioned problem, namely, significant increases in throughput. In the exposure process, electron beams are focused into rectangular patterns in certain sizes by means of an aperture, the thus focused patterns are all transferred to a wafer at a time, and those patterns are connected to one another to thereby transfer all circuit patterns onto a wafer.
FIGS. 1A to 1D are cross-sectional views of an aperture used in the above-mentioned exposure system, illustrating respective steps of a method of fabricating the aperture.
As illustrated in FIG. 1A, first and second silicon substrates 1a and 1b are adhered to each other to thereby form a wafer 1. The first and second silicon substrates 1a and 1b are adhered to each other at a boundary surface 2. Then, the wafer 1 is formed at a surface thereof having a plane azimuth of (100) with a desired pattern of opening 1c by conventional photolithography and etching. Then, a protection film 3 for protecting the wafer 1 from wet etching is formed all over upper, side and lower surfaces of the wafer 1 by chemical vapor deposition (CVD). For instance, the protection film 3 is a silicon nitride film.
Then, as illustrated in FIG. 1B, a photoresist film is formed on a lower surface of the wafer 1, and then, is patterned so that the photoresist film has an opening below the openings 1c of the wafer 1. Then, the protection film 3 is dry etched with the patterned photoresist film 4 being used as a mask, to thereby form a window 5 in the protection film 3.
Then, as illustrated in FIG. 1C, the wafer 1 is etched back at a lower surface thereof at an area exposed through the window 5. The wafer 1 is etched by means of a wet etching solution until the first silicon substrate 1a appears. Namely, the second silicon substrate 1b is etched by an entire thickness thereof as well as the boundary surface 2. Thus, there is formed an opening 6 at a lower surface of the wafer 1. For instance, there may be used heated alkaline solution such as potassium hydroxide and hydrazine, as the wet etching solution.
Since a plane azimuth (111) appears in the opening 6 during the wet etching, the opening 6 can be designed to have a tapered wall.
Then, as illustrated in FIG. 1D, the patterned photoresist film 4 and the protection film 3 are all removed. Thereafter, an electrically conductive film 7 is formed on an upper surface of the wafer 1 by sputtering in order to prevent charge-up which would occur when electron beams are radiated onto an upper surface of the wafer 1. For instance, the electrically conductive film 7 is composed of Au.
Thus, there is completed an aperture 10 as illustrated in FIGS. 2A to 2C, wherein FIG. 2A is a cross-sectional view of the aperture 10, FIG. 2B is a top plan view of the aperture 10, and FIG. 2C is an enlarged view of the opening 6. As illustrated in FIG. 2B, the opening 6 formed in the aperture 10 consists of smaller openings 6a arranged in 4.times.3. Each of the openings 6a is designed to have a pattern as illustrated in FIG. 2C. That is, each of the openings 6a includes a plurality of horizontally extending slits 6b in parallel with one another. When electron beams are radiated to a photoresist film through the opening 6 having a plurality of the smaller openings 6a each of which includes the slits 6b, portions of the photoresist film in alignment with the slits 6b are exposed, and hence, such portions of the photoresist film are etched out, or are not etched out.
The thus completed aperture 10 is fixed onto a holder 20, as illustrated in FIGS. 3A and 3B. The holder 20 is constructed as a frame having a central window 21 through which the opening 6 of the aperture 10 is exposed, and has a stepped portion 22 having a reduced thickness, around the central window 21. The stepped portion 22 has an upper surface as an adhesion surface 23 on which the aperture 10 is adhered at its marginal portion by means of an adhesive layer 30 composed of silver paste, for instance. For instance, the aperture 10 is fixed onto the stepped portion 22 of the holder 20 by first applying the adhesive on the adhesion surface 23, and compressing the aperture 10 onto the stepped portion 22.
The apparatus including the aperture 10 fixed onto the holder 20, which has been explained with reference to FIGS. 1A to 3B, has a problem that a portion 30a of the adhesive 30 may be forced out into the central window 21 of the holder 20, or a portion 30b may be forced out onto an upper surface of the aperture 10, passing between the aperture 10 and the holder 20. Those portions 30a and 30b of the adhesive 30 contaminates the apparatus, and causes problems of reduction in an accuracy of painting with electron beams, and reduction in a lifetime of the aperture 10.
In particular, if the portion 30a of the adhesive 30 is forced out into the central window 21 in a significantly excessive amount, the portion 30a may disadvantageously reach the opening 6 of the aperture 10, and may partially cover the slits 6b of the opening 6 therewith, as illustrated in FIG. 4B. If electron beams are radiated onto a photoresist film with the slits 6b being covered with the adhesive 30b, there would be generated a defect 31 in a photoresist pattern PR, as illustrated in FIG. 4C.